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R. C. Canfield, R. J. Leamon, Z. Blehm (Physics Department, Montana State University), A. A. Pevtsov (National Solar Observatory, Sunspot)
Observers see ample evidence of helical structures in erupting solar filaments, X-ray sigmoids and CMEs. It has been argued that the total amount of twist in a given loop is a factor in its MHD stability [Priest, 1984]. A simple model illustrates this point. Consider a cylindrical force-free magnetic field with constant \alpha = T /L, where L is the length of the tube and T is the total twist contained within it. The tube is stable to the MHD kink instability for total twist below a critical value Tc ~2 \pi. Rust and Kumar [1996] compared the shape of 49 transient, bright sigmoid structures to the signature of a helically kinked flux rope. From a study of the aspect ratios of these transient sigmoid brightenings, they inferred that the cause of CMEs is the eruption of an unstable, kinked magnetic field.
We have analyzed 191 X-ray sigmoids in the the Yohkoh SXT data, measuring the angle \gamma at which the sigmoid crosses its central axis and the length of the sigmoid along that axis (which is not identical to L, but is closely related to it). In a simple 2D force-free analysis, Pevtsov et al.\ [1997] showed that \alpha = ( \pi / L ) \sin \gamma, implying that \sin \gamma is a measure of the total twist T. By simple visual inspection of the Yohkoh SXT movies, we have identified well-known signatures of eruption, i.e., X-ray cusps and arcades. We find no relationship between the frequency of occurrence of such signatures of eruption and \sin \gamma.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.